Condensation Particle Counters (CPCs) have traditionally had high false-count rates, especially when compared with optical particle counters. In an optical particle counter, false counts are usually caused by optical or electrical noise, and can often be filtered out or eliminated because the false counts have scattering characteristics that create pulses that look different than pulses from real particles. In a CPC however, false counts are caused internal to the CPC when particles are formed within the instrument. Since these particles then grow the same as the particles being measured, the false-count particles look the same as the real particles, thereby making it difficult or impossible to distinguish between real counts and false counts. Consequently, eliminating false counts in a CPC is an exceedingly difficult task.
Moreover, the high false-count rates are typically more prevalent in water-based CPCs as compared with CPCs based on other types of working fluids (e.g., alcohol based CPCs using, for example, isopropanol or butanol). Previous attempts by various manufacturers to develop a high flow rate CPC with a low false-count rate for use in cleanroom applications have been unsuccessful. While some approaches yield very good false-count rates initially, these approaches have been unable to sustain these low false-count rates for significant lengths of time.
Since CPCs have been typically used to measure higher concentrations of particles, this high false-count rate has not been an issue in most applications. However, the high false-count rate is a significant problem for measuring low particle concentrations as found in, for example, cleanrooms and environments in which electronics manufacturing processes occur. The high false-count rate becomes even more critical with the increased sample flow rate of a cleanroom CPC, which is typically 2.83 liter per minute (0.1 ft3 per minute).
However, as volumetric sample flow rates increase, any working fluid that drains in to the flow path has a tendency to create bubbles or other forms of small droplet. The small droplets then grow into large particles that are detected by an optical particle detector within the CPC. Since these counts are generated internally to the CPC and are not caused by actual particles from a monitored environment, the internally-generated counts are considered “false-particle counts” and occur even when the particle counter is sampling clean HEPA-filtered air.
The performance of a CPC is rated by the number of false counts over a specified time period. For example, a semiconductor cleanroom may require less than six false counts per hour. Consequently, in general, the lower the number of false counts, the better the instrument. The disclosed subject matter describes techniques and designs to reduce or eliminate false-particle counts in a CPC.